1. Field of the Invention
The present invention relates to paperboard food packages. More particularly, the invention relates to a process for heat sealing paperboard food packages and the packages produced by the process.
2. Description of the Prior Art
One or two serving portions of precooked and/or frozen food are frequently packaged for consumer distribution in paperboard trays. Such trays are folded from preprinted and die-cut paperboard blanks.
Lids for these paperboard trays may take one of several forms including a lid that is an integral continuation of the same paperboard blank from which the tray is erected, such a lid flap being crease hinged to one sidewall of the tray. Another type of lid is an independent paperboard blank that is adhesively secured or plastic fuse bonded to a small perimeter flange folded from the upper edge of the tray sidewalls.
To protect the food contacting surfaces of the paperboard tray and lid from moisture penetration, the internal surfaces are coated with one or more continuous films of extruded thermoplastic. This internal moisture barrier is applied as a free-flowing curtain to the paperboard while still in the continuous web form and, prior to printing and cutting the paperboard into blanks. Low-density polyethylene (LDPE), polypropylene (PP) and polyethylene terephthalate (PET) are merely three of the more popular thermoplastic polymers used for this purpose.
However, if the paperboard blanks have closures which require the bonding of a polymer coated interior surface to a non-polymer coated exterior surface, it may be necessary to also coat the exterior of the tray and lid with an external coating of polymer for the primary function of heat sealing these surfaces to one another.
As with all conventional fourdrinier laid paper webs, bleached paperboard is produced with a "felt side" and a "wire side." For food packaging, the felt side is preferred for the thermoplastic food moisture barrier. This felt side becomes the interior of a food package formed from the paperboard. The wire side is given a clay coating that is smoothed by calendering to provide a superior print surface. The wire side becomes the exterior of a food package formed from the paperboard. However, the clay coated paperboard surface will not readily adhere to a surface having a polymer coating. Consequently, it is difficult to secure a reliable heat seal between an interior thermoplastic surface of a paperboard package and the exterior clay coated surface as described above.
A specific example arises with respect to a tray having an integrally hinged lid with flaps extending from the lid for overlapping three tray sidewalls. The tray contents are tightly enclosed by bonding the interior surfaces of the polymer coated lid flaps against the exterior clay coated surfaces of the three tray sidewalls. Production line pace permits less than 1 second to secure the flap-to-wall bonds. The present invention focuses upon the mechanics of these bonds.
As previously inferred, the predominately practiced prior art avoids the polymer-to-clay surface bonding problem by curtain coating the entire clay-coated wire side of the paperboard web with an extruded film of an extremely hot, viscously flowing polymer, in the same manner as the food moisture barrier is applied to the paperboard web felt side. Frequently, the clay surface of the wire side is pretreated by open flame or corona discharge immediately prior to hot polymer contact to enhance deep bonding of the polymer with the topically applied clay and underlying paper fiber. By this expedient, both the underside surfaces of the tray lid flaps and the exterior surfaces of the tray sidewalls are coated with the same or at least compatible polymers and are easily fuse bonded at a low tack temperature and a low joint compression pressure.
However, polymer coating an entire exterior package surface, for the singular reason of effecting a reliable, rapid and low pressure heat seal between the tray side walls and the lid closure flaps, produce several collateral consequences that must be accommodated. The first and obvious consequence is economic. Proportionately, the exterior polymer coat adds significantly to the total package cost. Moreover, only a small proportion of the full exterior surface coating is actually functional for the purpose for which it is applied. A second, adverse consequence is caused by the poor ink receptivity by the exterior polymer coating which necessitates the use of specialized, volatile inks for the colorful sales and informational graphics applied to the exterior surfaces of these packages. Use of these inks creates additional economic, safety and environmental concerns. Also, the exterior polymer coat is softer than the clay coat and, therefore, mars and scratches more easily. Finally, the additional polymer coating further complicates beneficial disposal of the used packaging by composting or repulping and recycling of the fiber composition of the package.
As additional factors to the present invention's prior art and development, it should be understood that a typical commercial food tray filling line advances at the rate of 60 to 120 units per minute. Consequently, any step or process in the continuous production line that requires a full stop of the subject unit must be accomplished in one second or less. Other processing steps are performed on a moving unit. Moreover, once the tray is filled with a food product and the lid positioned, the tray's inside surfaces are inaccessible for nip pressure backing. Any force applied to a lid flap for sealing against a tray side wall must be less than the crushing capacity of the erected tray. Frequently, only a gentle touch is permissible.
In addition to the previously described expedient of heat fusing polymer coatings applied to both paperboard surfaces, the prior art has also relied upon both cold set and hot set adhesives to achieve lid-to-tray seals. However, each of these adhesive sealing devices carry respective adverse consequences. Cold set adhesives are extremely slow setting and, therefore, incompatible with a production capacity of 120 units per minute. Hot melt adhesives have relatively low softening temperatures which are incompatible with typical oven temperatures used when the food within the package is heated for consumption.
It has previously been suggested that paperboard cartons of the type disclosed herein may be successfully sealed with the use of water based polymer adhesives as the sealing means. Exemplary prior art paperboard cartons are disclosed in commonly assigned U.S. Pat. Nos. 5,217,159 ('159) and 5,228,272 ('272) both to B. G. Calvert et al., respectively entitled "Heat Sealed Paperboard Carton Having Polymer Coating on One Side Only" and "Product and Process for Heat Sealing a Paperboard Carton Having Polymer Coating on One Side Only." The water based polymer adhesives of the ('159) and ('272) patents perform particularly well when the exterior of the carton blanks are printed with water based inks. However, when the carton blanks are printed with organic solvent based inks, the organic solvent base adhesives of the present invention are preferred. The use of an organic solvent based adhesive under such circumstances improves the wettability of the printed surfaces thereby increasing bonding strength, and yields reduced drying loads on the press as compared with water based adhesives. Moreover, organic solvent based adhesives are preferred for cartons used to package frozen foods where the water based adhesives are susceptible to cracking. Finally, the use of water based adhesives do not provide the manufacturer of such cartons with as great of a variety of coatings with higher solids that the organic solvent based adhesive solutions can provide.
It is apparent therefore, from the above, that there exists a need in the art for a paperboard carton which is coated with a polymer coating on one side only and which relies on a solvent based adhesive applied in specific patterns to provide a means of sealing. It is the purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.